There is widely known a gas engine system in which air supplied via a turbocharger and fuel gas are mixed, the mixture is supplied into a combustion chamber, and the mixture is ignited and combusted in the engine.
In the gas engine system, in a case where the gas engine is started from a state where the gas engine is cooled due to a low outside air temperature as in, e.g., wintertime, when outside air is used as the air mentioned above, the low-temperature outside air is sucked into the gas engine at the time of start of the gas engine, and hence there is a problem that the startability of the engine is reduced.
To cope with this, Patent Document 1 discloses a technology capable of preventing the reduction in startability in the case where the gas engine is started from the state where the gas engine is cooled due to the low outside air temperature as in wintertime. The technology disclosed in Patent Document 1 relates to the gas engine system in which the engine has a passage configured to guide outdoor air (outside air) into the engine and a passage configured to guide indoor air (inside air) into the engine, only the inside air is supplied into the engine and is also heated when the load of the engine is less than a specific value, and only the outside air is supplied into the engine when the load of the engine is not less than the specific value.    Patent Document 1: Japanese Patent Application Laid-open No. 2003-83181
Incidentally, in the gas engine system, there are cases where unpurified gas represented by VAM (Ventilation Air Methane) discharged from a mine (hereinafter referred to as unpurified gas) is used as air and fuel supplied to a turbocharger. The unpurified gas often contains a solid impurity such as dust or the like and, particularly when the unpurified gas is used as air and is supplied to the turbocharger together with outside air, the impurity causes the wear of a device or the abnormal combustion in the engine so that it is necessary to remove the impurity.
However, in the technology disclosed in Patent Document 1, as described above, although it is possible to suppress the reduction in startability in the case where the gas engine is started from the state where the gas engine is cooled due to the low outside air temperature as in wintertime, no consideration is given to a case where the unpurified gas is supplied to the turbocharger together with the outside air. As a result, when the technology disclosed in Patent Document 1 is applied to the case where the unpurified gas is supplied to the turbocharger together with the outside air, it is not possible to remove the impurity and there is a possibility that the impurity flows into the engine and the engine does not operate normally.